Aluminum oxide, tantalum oxide and niobium oxide have conventionally been used as dielectric materials of electrolytic capacitors. In addition, studies have long been conducted on capacitors using titanium dioxide, which has a larger relative dielectric constant than the above oxides, for the dielectric material (hereafter, frequently referred to as a “titanium capacitor”). However, due to the problem of large leakage current, these titanium capacitors have yet to be put to practical use. This problem of large leakage current is critical, especially for the metallic electrodes effective in reducing the extent of impedance in the gigahertz region which has been required recently, since restoration (that is, reoxidation) of electrical leakage, which takes place when an electrolyte solution or electrically conductive polymer is used for the cathode in an electrolytic capacitor, cannot be expected.
The following describes previous attempts made to reduce the level of leakage current in titanium capacitors.
For example, in Patent Document 1, although a non-aqueous solvent is subjected to anodic oxidation for use as an electrolyte solution, there is a description stating that “a product chemically converted in a non-aqueous solution instantly deteriorates when transferred to an aqueous solution”. Consequently, although not described in Patent Document 1, it is clear that a non-aqueous solvent having low electrical conductivity is used as an electrolyte solution. In addition, possibly due to the use of a non-aqueous solvent as an electrolyte solution, although the leakage current is low, dielectric loss tangent at 10 kHz is 10% or more in all cases.
Further, in Patent Document 2, a method has been shown for obtaining an anodized film having superior electrical properties by using a titanium alloy containing vanadium, chromium and aluminum. However, the dielectric loss tangent thereof is 1.5% or more.
Moreover, in Patent Document 3, there is a description stating that a capacitor obtained by anodic oxidation of titanium has leakage current that is greater than that of tantalum or aluminum by two digits or more. Furthermore, in Patent Document 3, a method has been shown for reducing leakage current by forming a passive layer with a nitric acid solution as a pretreatment of anodic oxidation. However, the dielectric loss tangent of the resulting sample is 1.5% or more.
Also, in Patent Document 4, the same inventor as that of Patent Document 3 has shown that the addition of tungsten or molybdenum to titanium reduces the level of leakage current to about one-half, as compared to the cases where no addition was made. However, even though the leakage current problem is improved by reducing the level thereof down to a degree of ½, this level is still inadequate for practical use.
In addition, Patent Document 5 has shown that leakage current and loss can be reduced by containing barium peroxide or strontium peroxide in a molten salt of sodium nitrite and anodizing at a temperature of 280 to 350° C. However, the dielectric loss tangent at this time is 2.8% or more.
Further, Patent Document 6 has shown that leakage current is reduced by using an alloy containing 20 to 30 atomic % of aluminum in titanium. However, in Patent Document 6, measurement of electrical properties has been carried out in an electrolyte solution. It has generally been known that in electrostatic capacitors, an electrical leakage portion is reanodized and insulated (restoration effects) when a direct current voltage is applied in an electrolyte solution, electrically conductive polymer and the like. Therefore, it is assumed that the level of leakage current was reduced due to the restoration effects in the measurement made in Patent Document 6.
In addition, in Patent Document 7, a method has been shown for obtaining a capacitor having a satisfactory dielectric loss tangent by adjusting the anodic oxidation conditions and carrying out a heat treatment thereafter. In Patent Document 7, there is a description stating that low temperatures are more desirable for anodic oxidation, and a case where an anodic oxidation process was carried out at a temperature of 5° C. is disclosed therein as an example. However, although the electrical properties of capacitors were measured in Patent Document 7 by employing an electrolyte solution having a restoration capacity as a cathode for the capacitors, the dielectric loss tangent thereof exceeded 0.6%.
Further, in Non-Patent Document 1, it has been shown that the dielectric constant of an anodized film of titanium is dependent on the temperature for anodic oxidation. However, the dielectric constant reduced as the temperature decreased, and the relative dielectric constant at 303 K (that is, 30° C.) was 26.2. It is difficult to produce a capacitor having a large capacity with this dielectric constant.    [Patent Document 1] Japanese Examined Patent Application, Second Publication No. Sho 33-5816    [Patent Document 2] U.S. Pat. No. 3,126,503    [Patent Document 3] Japanese Examined Patent Application, Second Publication No. Sho 42-27011    [Patent Document 4] Japanese Examined Patent Application, Second Publication No. Sho 42-24103    [Patent Document 5] Japanese Examined Patent Application, Second Publication No. Sho 43-2649    [Patent Document 6] Japanese Examined Patent Application, Second Publication No. Sho 54-1020    [Patent Document 7] Japanese Unexamined Patent Application, First Publication No. Hei 5-121275    [Non-Patent Document 1] Corrosion Science, Vol. 37, No. 1, pp. 133-144